The present application is based on Japanese priority application No. 2000-301490 filed on Sep. 28, 2000, the entire contents of which are hereby incorporated by reference.
This invention generally relates to semiconductor devices and especially to an optical semiconductor device that extracts optical clock signals from an optical signal.
In the field of optical telecommunication technology, it is general to superimpose an optical clock signal to optical signals that are transmitted through an optical fiber. Thus, the optical clock signals have to be reproduced from the received optical signal in repeater devices or reception devices that are connected to the optical fiber.
FIG. 1 shows the construction of a known clock-extracting optical-detection device 100 used for reproducing optical clocks from such an optical signal.
Referring to FIG. 1, the clock-extracting optical-detection device 100 includes an optical coupler 11 coupled optically to an optical fiber 101 that transmits an optical signal having a frequency f0 from an input end 101A to an output end 101B, and the optical signal in the optical fiber 101 is branched by the optical coupler 11 and are introduced into an optical modulator 12. The optical modulator 12 is driven by a driving signal having a frequency of fclk from a voltage-controlled oscillator 16 and modulates the optical signal that has been branched by the optical coupler 11. The optical signal thus modulated by the optical modulator 12 is then detected optically in an optical detector 13, and an output signal of frequency f0-mfclk is obtained.
The output signal of the optical detector 13 is then supplied to a phase comparator 15 for phase-comparison with a reference frequency signal supplied from a reference signal source 14 with a frequency f1. The phase comparator 15 thereby produces a voltage signal representing the phase difference between the output signal of the photodetection circuit 13 and the reference frequency signal, and the voltage signal thus produced is supplied to a voltage-controlled oscillator 16. Thus, the frequency fclk of the driving signal is controlled so as to minimize the foregoing phase difference. In other words, the phase comparator 15 performs a feedback control of the voltage-controlled oscillator 16.
As a result of such a feedback loop operation, the phase difference is controlled to substantially zero, and the output frequency signal 102 of the voltage-controlled oscillator 16 in this state is taken out as the clock signal that is synchronized with the optical clock in the optical signal.
Conventionally, the optical modulator 12 and the optical detector 13 have been formed as individual components in such a clock-extracting detection device 100. The optical modulator 12 and the optical detector 13 have been connected by an optical fiber. However, such a construction is bulky and fragile, and has a problem of difficulty in realizing optical coupling without causing optical loss.
In view of the foregoing drawbacks of conventional construction, it is conceivable to form the optical modulator 12 and the optical detector monolithically on a common substrate in the form of integrated optical modulator/detector and to connect these units by a waveguide formed also monolithically on the common substrate. However, due to the inherent difference between the requirements imposed to an optical detector and the requirements imposed to an optical modulator, it is difficult to realize an efficient optical coupling between these units, in spite of the fact that both the optical modulator 12 and the optical detector 13 are formed based on a waveguide structure. Because of this reason, such a construction has not actually been attempted.
In the clock-extracting optical-detection device 100, it should be noted that the optical modulator 12 has an active layer that forms a part of the optical waveguide. Thereby, a refractive-index change or optical absorption is induced in the active layer in response to a voltage signal, and the optical beam propagating through the active layer undergoes optical modulation. On the other hand, the optical detector 13 has an optical absorption layer and detects the optical signal by detecting the optical carriers that are produced in response to absorption of the incoming optical beam by the optical absorption layer. Thus, in the event the optical waveguide is used to connect the optical modulator 12 and optical detector 13 formed monolithically on a common substrate, it is necessary that the optical waveguide achieves an efficient optical coupling both to the active layer of the optical modulator and the optical absorption layer of the optical detector.
Meanwhile, in the case of constructing the optical modulator 12 by a high-speed optical interferometer of Y-type or Mach-Zehnder-type, it is an indispensable condition that the optical modulator 12 performs a single mode operation for realizing a satisfactory extinction ratio.
FIG. 2 shows the relationship between the thickness d and width W imposed for the active layer of the optical modulator 12.
FIG. 2 is referred to.
Designating the curve shown in FIG. 2 as f(W), it should be noted that the active layer of the optical modulator 12 forms a single-mode waveguide when the condition d less than f(W) is met. It functions as a multi-mode waveguide when the condition d greater than f(W) is met. Thus, in the case of operating the optical modulator 12 in single mode, it is desirable and necessary that the thickness d is increased when the width W is small and is decreased when the width W is large. As long as the relationship of FIG. 2 is maintained, the width W and the thickness d may be chosen appropriately so as to facilitate the fabrication process of the optical modulator 12.
On the other hand, FIG. 3 shows the relationship between the operational frequency band f of the optical detector and the thickness d.
As can be seen from FIG. 3, the distance, and hence the time, for an optically excited carrier to move through the optical absorption layer and reach an electrode is increased when the thickness of the optical absorption layer is large. Thereby, the operational frequency band f of the optical detector becomes inevitably low. Thus, it is preferable to reduce the thickness of the optical absorption layer from the viewpoint of improving the response characteristics of the optical detector. In the optical detector, it should be noted that the optical beam is not needed to be a single mode beam in the optical absorption layer.
In the event the thickness of the optical absorption layer is small like this, on the other hand, it is not possible to provide sufficient photodetection sensitivity, unless the length L of the optical absorption layer shown in FIG. 4 is increased so that the optical beam is absorbed sufficiently. Alternatively, the width has to be increased so that the cross-section area of the optical absorption layer is increased. To increase the cross-section area of the optical absorption layer without increasing the thickness, there is no way but to increase the width of the optical absorption layer. However, such an increase of width of the optical absorption layer invites an increase of area of the optical absorption layer and associated increase of parasitic capacitance. When there occurs such an increase of parasitic capacitance, there occurs a decrease of operational frequency band f of the photodetector 13 as shown in FIG. 5. Associated with this, the response speed is decreased. Thus, the optical absorption layer of the optical detector has to be designed in view of the relationship of FIGS. 3-5 such that the frequency band and the photodetection sensitivity are both optimized. The restriction thus imposed on the design of the optical detector 100 is stricter than the case of optimizing the shape of the active layer of the optical modulator 12.
Thus, the requirement imposed on the optical absorption layer of the optical modulator 12, especially on the aspect ratio thereof, is different from the requirement that is imposed on the aspect ratio of the optical absorption layer of the optical detector. Because of this, no desirable optical coupling is achieved when these devices are simply connected by an optical waveguide, and it has been inevitable to suffer substantial optical loss in the event the optical modulator 12 and the optical detector 13 are to be formed on a common substrate in the construction of FIG. 1.
Accordingly, it is a general object of the present invention to provide a novel and useful optical semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide an optical semiconductor device in which an optical modulator and an optical detector are integrated on a common substrate in a state of being coupled optically with high efficiency.
Another object of the present invention is to provide an optical semiconductor device, comprising:
a substrate;
an optical waveguide formed on said substrate, said optical waveguide constituting an interferometer-type optical modulator;
an optical absorption layer formed on said substrate in optical coupling with said interferometer-type optical modulator, said optical absorption layer forming an optical detector; and
an optical-spot conversion part interposed on said substrate between an output end of said optical waveguide and an input end of said optical absorption layer, said optical-spot conversion part converting a spot radius of an optical beam between said interferometer-type optical modulator and said optical detector.
Another object of the present invention is to provide an optical semiconductor device, comprising:
a substrate;
an optical waveguide formed on said substrate, said optical waveguide constituting an interferometer-type optical modulator; and
an optical absorption layer formed on said substrate in optical coupling with said optical waveguide, said optical absorption layer forming an optical detector.
According to the present invention, the optical modulator and the optical detector are formed monolithically on a common substrate. Further, an optical-spot conversion part is formed monolithically between the optical modulator and the optical detector. As a result, it becomes possible to implement clock extraction and optical detection with high reliability while using a simple construction. According to the present invention, a high optical coupling is guaranteed between the optical modulator and the optical detector while using a simple construction.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.